Re: [boost] Re: dynamic_void_cast

From: Terje Slettebø (tslettebo_at_[hidden])
Date: 2002-12-10 08:18:04

>From: "Johan Nilsson" <johan.nilsson_at_[hidden]>

> The hack makes no 'fixed' assumptions on binary object
> layout, rather, it relies on the fact that any polymorphic type can be
> queried for an implemented interface (aka 'base class'). It does, however,
> make the assumption that the location of the rtti itself data is fixed for
a
> specific c++ implementation across different polymorphic types.
>
> Sample usage (demonstrating syntax only):
> --------------
>
> void foo(void*)
> {
>     try
>     {
>         IMyBaseClass* pmc = dynamic_void_cast<IMyClass>(pv);
>
>         if (pmc)
>        {
>             pmc-MyFn();
>         }
>     }
>     catch (std::exception&)
>     {}
> }
>
>
> The hack:
> ------
> template<typename T>
> T* dynamic_void_cast(void* pv)
> {
>   struct rtti_obj__
>   {
>     virtual ~rtti_obj__() = 0;
>   };
>
>   rtti_obj__* pro = static_cast<rtti_obj__*>(pv);
>
>   try
>   {
>     return dynamic_cast<T*>(pro);
>   }
>   catch (bad_cast&)
>   {
>     throw;
>   }
>   catch (exception& e)
>   {
>     throw bad_cast(e.what());
>   }
> }
>
> ----------

Why do you catch, and just rethrow exceptions? Why not just let them
propagate (no try/catch), or turning them into return zero? Also, what other
exception, besides bad_cast, can dynamic_cast throw, which necessitates the
second catch clause?

Regards,

Terje